Since CVT of the traction drive type for automobiles has a great capacity of torque transfer and the condition in the use is severe, it is essential that a traction oil used for CVT has a traction coefficient sufficiently greater than the value prescribed in the design of CVT at the lowest temperature in the temperature range of the use, which is a high temperature (140° C.).
On the other hand, a small viscosity even at −40° C. is required for starting an engine at low temperatures in cold areas such as northern America and northern Europe. However, the traction coefficient at high temperatures and the property for starting an engine at low temperatures are contradictory properties. A base oil for a traction oil satisfying both of these contradictory properties at a high level has been required.
Moreover, excellent viscosity-temperature characteristics are also essential for practical applications in combination with the small viscosity.
Under the above circumstances, the present inventors discovered a high performance base oil for a traction oil exhibiting excellent properties at high and low temperatures which were not achieved before (Japanese Patent Application Laid-Open No. 2000-17280). This base oil for a traction oil has advantageous properties in that the traction coefficient at high temperatures is greater and the viscosity at low temperatures is remarkably improved in comparison with those of a commercial base oil which is 2,4-dicyclohexyl-2-methylpentane. However, a further improvement in the viscosity at low temperatures have been desired so that the property for starting an engine at low temperatures is further improved.
As the base oil having a small viscosity which is added to the above high performance base oil for traction oils and improves the fluidity at low temperatures without decreasing the traction coefficient at high temperatures, the present inventors have developed a group of compounds having specific structures and exhibiting a viscosity index of 0 or greater by the improvement of the bicyclo[2.2.1]heptane hydrocarbon compound which had been discovered by the present inventors (Japanese Patent Application Publication Heisei 5(1993)-63519).
Under the above circumstances, the present invention has an object of providing a fluid for traction drives for automobiles exhibiting a great traction coefficient at high temperatures which is important for practical application to CVT for automobiles and improved fluidity at low temperatures, i.e., small viscosity at low temperatures, which is important for starting engines at low temperatures.